Field of the Invention
This invention relates to DC power connecting circuits, and, more particularly, to switching circuits adapted to switchably deliver power from a low power source to a plurality of circuits so as to match the respective circuit requirements to the source characteristics. In the case of low voltage level DC sources, the power connecting circuit may also comprise one or more converter circuits for raising the delivered voltage level.
The switching apparatus of this invention is illustrated in the preferred embodiment of a cardiac pacer. Lithium-iodide battery cells are now available and in wide use in the field of cardiac pacers, and such cells offer promise of a substantially greater lifetime than battery cells in prior use in this field. However, when the characteristics of such battery cells are matched with the electrical requirements of pacers, it is seen that for most pacers the full potential of the newer cells simply is not available, primarily because after a certain time period the changing characteristics of the cells produce an altered pacer operation. If such altered operation falls below allowable limits, the pacer has to be removed from the patient even though it may have an appreciable amount of energy left within it. In the pacer embodiment, as well as in other uses to which this application is directed, an internal characteristic of the power source, e.g., internal impedance, varies with energy depletion, which variation has consequences upon the operation of at least a given circuit portion of the apparatus, and which must be accounted for. In many cases the exact variation of the power source characteristics as a function of energy depletion will not be known precisely, even though the approximate nature of the variation can be anticipated. It is frequently desirable, if not necessary, to isolate the source sensitive circuit portion from the remaining portion, and to provide that the source sensitive portion not be affected by the loading action of the remaining portion. While the invention is here illustrated in the preferred embodiment of a cardiac pacer utilizing a lithium-iodide cell, it is to be understood that the invention has a scope which embraces any device utilizing a power source which has changing characteristics with time, which power source drives at least two circuit areas, each of which has a different dependence upon the characteristics of the circuit source.
A first primary characteristic of the lithium-iodide cell is that its maximum voltage output is 2.8 volts. The chemical nature of such a cell provides that in terms of electrical characteristics it presents an ideal 2.8 volt source, in combination with an internal resistance. While the 2.8 volt source remains a fixed constant due to the chemistry of the cell, the internal resistance may and indeed does vary as a function of energy delivered by the battery, i.e., its energy depletion. Roughly speaking, such internal resistance increases linearly with energy depletion until a knee in the curve is reached wherein the internal resistance rises dramatically, at which point the useful life of the source is over very soon. However, if in fact the cell can be utilized throughout the full extent of the linear range of relatively low resistance, and utilized throughout the full extent of this range, then its lifetime can be maximized. It is noted that in this specification the example of the lithium-iodide cell is used for illustrative purposes. For purposes of brevity, the term lithium cell, or lithium battery, is used to denote lithium-iodide, lithium-silver chromate, and other cells of such class. It is to be noted, however, that other types of cells are included as well within the invention, and while they may have differing characteristics, the basic characteristics of the cells are sufficiently similar such that the principles of this invention are applicable. For the lithium-iodide cell, a top voltage of 2.8 volts means that for practical purposes a voltage converter device is required in order to raise the available voltage level to a level which provides a safe voltage for circuit operation. While many modern electronic circuits can be designed to operate at less than 2.8 volts, such a requirement imposes severe design limitations, which limitations drastically increase the required complexity of the pacer circuitry and consequently reduce the resulting reliability. Certain circuits simply require a minimum voltage, such as some CMOS devices which require at least 2.4 volts for operation. Even though such circuits could in fact be utilized, the margin of safety would be virtually negligible, such that a relatively small percentage of energy depletion of the battery source would reduce the voltage to a point where reliable circuit operation would no longer be expected.
Another fixed parameter of present day pacer design is that the pulse generator, or oscillator, which provides the stimulus pulses is inherently a voltage sensitive circuit. By that it is meant that the frequency and/or the pulse width of the oscillator vary as a function of the voltage supply. This is the case both because the oscillator circuits which have been found to be reliable for pacer use are of this nature, and because of the presently accepted philosophy of designing the oscillator to be voltage sensitive so as to produce an indication of the battery condition. In accordance with this philosophy, most pacers have provision for external monitoring of the pacer, such as by magnetic coupling devices, whereby the patient or a physician determines the pulse repetition rate and from this information obtains a reading of the depletion condition of the battery. This design feature is sufficiently widely accepted that it can be stated that the voltage sensitive oscillator circuit is at the present a fixed parameter in the pacer industry.
Considering the above factors which influence the design of a pacer, and seeking to determine a design for best utilization of a lithium type battery source, it is noted that conceptually the pacer circuitry can be divided into a first source sensitive, or voltage sensitive circuit which includes the pacer oscillator, and a second circuit which is not as critically sensitive to the source output, or source condition, such second circuit portion including the output pulse circuit. In a typical example, the steady state current drain for an entire pacer may be about 25 microamps, such drain being accounted for by approximately a 5 microamp current drain through the amplifier, logic and oscillator portions of the pacer circuitry, and the remaining 20 microamps being accounted for by the stimulus pulse outputs. The output circuit thus represents a relatively high power drain to the battery, and when it draws current it acts to produce a voltage drop across the internal resistance of the source, which in turn causes a corresponding voltage drop in the voltage level delivered to the low current drain voltage sensitive portion of the pacer. The problem is that the loading of the source caused by the output circuit produces a voltage drop which is relatively unacceptable when applied to the voltage sensitive circuit. In conventional designs the voltage delivered to the output circuit is the same as to the oscillator circuit, and may be permitted to drop to a certain level without dangerous impairment of pacer operation. Indeed, many circuits are now being incorporated into pacers to lengthen the pulse width of the stimulus output to counteract decreasing pulse current. However, the loading effect of the stimulus pulses on the voltage supplied to the oscillator becomes more severe with battery depletion, and it is clear that there is a limit to the tolerable drop in oscillator frequency. Thus, conventional design provides that the oscillator frequency be the controlling variable, and the pacer is considered to be at the end of its useful life when its frequency drops by a predetermined amount. In fact, it is now understood that the battery source under such circumstances still retains a great amount of energy, and in particular an amount which could provide substantial continued operation. It is the failure of the circuit designs as used to date that leaves this residual amount of energy unused and that causes relatively premature removal of the pacer.
An alternate approach which has been considered as a solution to the above problem is to convert the available DC level to a level which is sufficiently high that it can be regulated to a somewhat lower level, the difference between the higher converted level and the regulated level providing a margin of safety which permits battery depletion while the voltage supplied to the circuitry is stabilized. For example, using this approach and a source which provides an initial voltage of 2.8 volts, the voltage level may be converted by a factor of 3 to about 8.4, and then regulated or stabilized down to about 6 or 6.5 volts. With this arrangement, until the effective battery output drops to the regulation voltage, no change is seen at the pacer circuitry. However, this arrangement has the distinct disadvantage that the regulator must dissipate a substantial amount of the battery energy in reducing the voltage to the stabilized level. In particular, and for the figures used in this illustration, at the initial starting of the lifetime of the pacer approximately 25 to 30% of the power being delivered by the battery would be dissipated across the regulator. Quite clearly, where extending the lifetime of the pacer is the design goal, it is not desirable to toss away this amount of power. In addition, this arrangement has the drawback that as long as the regulator is putting out a fixed voltage, there is no indication of battery change available, and only when the source is depleted to the point where the available voltage drops below the regulator voltage is there any indication. Of course, at the time that this indication is provided, then the patient is faced with a virtual crisis situation, since a new pacer must be implanted rather quickly.
In consideration of the above, it is seen that there is required a very thorough understanding of the optimum manner of utilizing the available energy from the newer longer life battery cells which are becoming available. What is called for is a circuit design which provides an optimum interface between the operating portions of the pacer circuitry and the battery, the interface being designed so as to provide for power delivery to the different circuit portions in a manner which optimizes the overall pacer operation.